1. Field of the Invention
This invention relates to the main unit of a key telephone system including a segment coder for companding (compressing and decompressing) a digital audio signal, and more particularly to a sound level control apparatus for use with an apparatus of the type described above.
2. Description of the Related Art
FIG. 1 shows a conventional monitor circuit of a digital audio apparatus.
Conventionally, a digital sound level control apparatus of the type mentioned above includes, as disclosed in Japanese Patent Application No. 72457/1986, entitled "Monitor Circuit" shown in FIG. 1, a circuit for adjusting the sound level output by sending a code signal to monitor body section 52 from sound level adjusting switch 54 and multiplying the PCM audio signal of the output of monitor signal extraction circuit 58 by the code signal to define the monitor sound level through sound level conversion memory 57, digital audio convertor 56 and analog amplifier 55.
The conventional digital sound level control apparatus includes a ROM in which a predetermined conversion table is stored. Accordingly, the conventional digital sound level control apparatus is disadvantageous in that much labor and time are required to write the conversion table into the ROM upon assembly of a digital key telephone system and in that, once the ROM is incorporated into a telephone apparatus, the sound level cannot be varied flexibly in conformity with the circuit with which the telephone apparatus actually is used.